Reciprocal changes in the firing probability of lateral and central medial amygdala neurons

Abstract

The amygdala is essential for classical fear conditioning. According to the current model of auditory fear conditioning, the lateral nucleus is the input station of the amygdala for conditioned auditory stimuli, whereas the central nucleus is the output station for conditioned fear responses. Yet, the lateral nucleus does not project to the central medial nucleus, where most brainstem projections of the amygdala originate. The available evidence suggests that the basal nuclei could transmit information from the lateral to the central medial nucleus. However, interposed between the basolateral complex and the central nucleus are clusters of GABAergic cells, the intercalated neurons, which receive inputs from the lateral and basal nuclei and contribute a massive projection to the central medial nucleus. Because it is impossible to predict the consequences of these connections, we correlated the spontaneous and auditory-evoked activity of multiple simultaneously recorded neurons of the lateral, basal, and central nuclei. The spontaneous activity of lateral and basolateral neurons was positively correlated to that of central lateral cells but negatively correlated to that of central medial neurons. In response to auditory stimuli, the firing probability of lateral and central medial neurons oscillated in phase opposition, initially being excited and inhibited, respectively. In light of previous anatomical findings, we propose that the lateral nucleus exerts two indirect actions on central medial neurons: an excitation via the basal nuclei and an inhibition via intercalated neurons.

Fig. 1.

Recording method and physiological identification of projection cells. A, Scheme illustrating one of the microelectrode configurations used to obtain simultaneous extracellular recordings of amygdala neurons. Only one of the three electrode rows is shown. Inset in top left shows a top view of the electrode array. The length of the different electrodes was adjusted to allow simultaneous recordings from the BL complex and central nucleus. The scheme shows the intended position of the microelectrodes at the beginning of the experiments. The array was lowered 80–160 μm before each recording session. B, Antidromic invasion of a CE_M neuron from the brainstem.C, Neuron of the lateral nucleus backfired from the perirhinal cortex. Data were digitally filtered (100 Hz to 10 kHz).Arrowheads indicate stimulation artifacts. Curved arrows in B1 and C point to spontaneously occurring action potentials colliding with antidromic responses. Note the fixed latency of antidromic spikes and their ability to follow high-frequency stimulation (B2).AHA, Amygdalo-hippocampal area; BL, basolateral nucleus; BM, basomedial nucleus;CE_M, medial sector of the central nucleus;CE_L, lateral sector of the central nucleus;CL, claustrum; LAT, lateral nucleus;OT, optic tract; PU, putamen;rh, rhinal sulcus.

Fig. 2.

Histological determination of recording and stimulating sites. A, Frontal section showing the traces left by one row of microelectrodes. Note that the plane of the section is not exactly parallel to the trajectory of the microelectrodes. For some electrodes, the last recording site was marked with small electrolytic lesions (arrowheads). Leftto Right, Arrowheads point to electrolytic lesions performed at the end of tracks through the CE_M, CE_L, and lateral (tworightmost lesions) nuclei. B,C, Photomicrographs in which arrowheadspoint to the traces left by the tip of stimulating electrodes just dorsal to the substantia nigra (B) and in the perirhinal region (C). A, Aqueduct; AT, anterior thalamic nuclei;BL, basolateral nucleus; BM, basomedial nucleus; CA, caudate nucleus;CE_M, medial sector of the central nucleus;CL, claustrum; H, hippocampal formation;L, lateral nucleus; LG, lateral geniculate nucleus; MG, medial geniculate nucleus;OT, optic tract; PP, pes pedunculi;PT, pretectal nuclei; PU, putamen;RE, reticular thalamic nucleus; rh, rhinal sulcus; V, ventricle.

Fig. 3.

Auditory-evoked responses in the central nucleus and BL complex. Population peristimulus histograms of auditory-evoked discharges recorded in various amygdala nuclei (indicated in thetop left of each panel). Each histogram was obtained by adding the response of several cells (n in top right of eachpanel) to two tones comprised between 4 and 10 kHz, each presented 20 times. Superimposed on each histogram is the auditory-evoked focal response picked up by the same electrodes during the unit recordings. The focal waves were digitally filtered between 3 and 50 Hz. In each graph, the left axis refers to the histogram and the right one to the amplitude of focal waves. Vertical and horizontal linesindicate the onset of the auditory stimuli and the average number of counts before stimulation, respectively.

Fig. 4.

Reciprocal changes in the firing probability of lateral and CE_M neurons during auditory stimulation. The peristimulus histograms of lateral (thick line) and CE_M (thin line) neurons shown in Figure 3were normalized so that the average bin values for the entire period is 100 and superimposed. A total of 31 CE_M and 184 lateral cells were used. Note that when the firing probability of lateral neurons increases, that of CE_M neurons decreases and vice versa.

Fig. 5.

Temporal relationship between the auditory-evoked activity of lateral and CE neurons. A total of 150 auditory stimuli, each lasting 1 sec, were presented (see Results). The activity of the simultaneously recorded neurons during each stimulus was cross-correlated. The resulting 150 cross-correlograms were added and normalized to the number of spikes in the reference cell. The population cross-correlograms were obtained by averaging the normalized cross-correlograms. The recording sites are indicated in the top left of each histogram. In each case, the leftnucleus corresponds to the reference cells and the rightone to the test cells. The number of cell couples (n), the number of spikes generated by the reference cells (nR), and the number of test cells (nT) are indicated in the top right of each histogram.

Fig. 6.

Temporal relationship between the spontaneous activity of lateral, BL, and CE neurons. Population cross-correlograms were computed for pairs of neurons recorded simultaneously in the sites indicated in the top left of each histogram. As in Figure 5, the left nucleus corresponds to the reference cells and the right one to the test cells. Spontaneous epochs were recorded in the waking state and lasted 2–3 min each. Periods contaminated by movements were not considered. Before averaging, the individual cross-correlograms were normalized to the number of spikes generated by the reference cell. The number of cell couples (n), the number of spikes generated by the reference cells (nR), and the number of test cells (nT) are indicated in the top right of each histogram.